1. Field of the Invention
The invention relates to a semiconductor device.
2. Description of Related Art
In order to make a trench gate type semiconductor device capable of withstanding high voltage, a thick insulation film is formed in a bottom portion of a trench in Japanese Patent Application Publication No. 2009-94484 (JP 2009-94484 A). This thick insulation film restrains leakage current, so that the semiconductor device becomes capable of withstanding high voltage.
In JP 2009-94484 A, a corner portion in a lower end of the gate electrode and a semiconductor substrate are isolated from each other by a relatively thin insulation film, similarly to a side surface of the trench. The insulation film is formed by oxidation of a semiconductor substrate, and, in the corner portion, the proportion of the semiconductor substrate oxidized is small. Therefore, in some cases, the insulation film does not sufficiently grow in the corner portion in the lower end of the gate electrode, which becomes a cause of an increase in the leakage current of the semiconductor device or a decline in the withstanding voltage of the semiconductor device.